1. Field of the Invention
The present invention relates to an electroluminescent element which consists of a transparent electrode and an opposed electrode, with a luminous layer interposed between them, so that application of an electric field across the two electrodes causes the luminous layer to glow.
2. Description of the Prior Art
Electroluminescent elements have been widely used as display elements and backlight of display devices for various kinds of machines and equipment. The electroluminescent element consists of a transparent electrode and an opposed electrode, with a luminous layer interposed between them, so that application of an electric field across the two electrode causes the luminous layer to glow. The luminous layer is made up of a phosphor layer formed by dispersing phosphor powder into a binder resin and a dielectric layer formed by mixing a highly dielectric fine powder (e.g., ceramics powder) into a highly dielectric binder resin.
The electroluminescent element constructed as mentioned above is produced in the following manner. At first, a phosphor paste is prepared by dispersing a phosphor powder into a solution of a binder resin in an organic solvent. This phosphor paste is applied onto a transparent electrode by screen printing or the like, followed by drying, to form a phosphor layer. Subsequently, a dielectric paste is prepared by dispersing a powder of ferroelectric substance (e.g., titanium oxide and barium titanate) into a solution of a binder resin in a solvent. This dielectric paste is applied onto said phosphor layer by screen printing or the like, followed by drying, to form a dielectric layer. Finally, a silver paste is applied onto said dielectric layer by printing technique to form an opposed electrode.
For an electroluminescent element to glow brightly, it is necessary that the phosphor layer have a high dielectric constant. To meet this requirement, it is necessary that the binder resin for the dielectric layer and phosphor layer have a high dielectric constant. An example of the binder resins having a high dielectric constant is a cyanoethyl resin such as cyanoethylcellulose. It is desirable for the improvement of luminance.
In the meantime, the phosphor powder in the phosphor layer should be protected certainly from moisture, because it is vulnerable to moisture and becomes deteriorated easily soon in a moist environment, greatly decreasing in luminance. For this reason, an electroluminescent element as a product is sealed in a sealing film; however, it is impossible to protect the phosphor powder completely from moisture by the sealing film. To supplement the insufficient moistureproofness of the sealing film, there has been proposed an idea of causing the binder resin to protect the phosphor powder from moisture. Unfortunately, the above-mentioned cyanoethyl resin is not desirable from the standpoint of moistureproofing function because it is poor is moisture resistance and gas barrier properties. Therefore, an electroluminescent element formed with cyanoethyl resin as the binder resin loses luminance in a short time when used in a high-temperature, high-moisture environment.
One binder resin desirable for protection of the phosphor powder from moisture is a fluoroplastic, which has superior moistureproofness and gas barrier properties. Unfortunately, a fluoroplastic resin does not increase luminance because it has a lower dielectric constant than a cyanoethyl resin. An additional disadvantage of a fluoroplastic is that it does not readily wet the phosphor powder and hence causes uneven glowing.